Cement-based infrastructure components maintenance works can be broadly classified into emergency/ad-hoc repairs and scheduled maintenance. The repairing work includes concrete infrastructures such as repair of pavement, sewerages, buildings and bridges, etc. In a background of high traffic demand, acute land constraints and growing community expectation in modern cities, all carriageway repair works are to be carried out under extremely tight working schedule and within limited hours up to at most a few days. Moreover, modern cities have complex sewage system with numerous sewer pipes constructed in the spaces running between infrastructures. Therefore, rapid hardening construction materials with specific performance requirement are urged for repair and maintenance of cement-based infrastructure components such as concrete carriageways and sewage system.
According to the nature of the major binding material adopted in different repair materials, they can be generally categorized into two systems, namely organic binder based system and inorganic binder based system.
One of the representatives of rapid repair material using organic binders is polymer concrete made by fully replacing hydraulic cement with polymeric binders or liquid resins such as thermosetting resins, vinyl monomers, and tar modified resins. They do not contain any cement hydrate phases in the binding system. Most of the thermosetting resin and monomer systems for polymer concrete are polymerized at ambient temperature. Fresh polymer concrete are placed and finished in a manner similar to conventional cement concrete. However, due to the short setting time, skillful labors are required to place, compact and texture the concrete. Besides, the stink odor and volatile organic compounds (VOCs) emission from the polymeric components during mixing and placing cause a great concern on health and environmental protection issues.
Ordinary Portland cement (OPC) is the most common inorganic binder based system used as rapid repair material. Utilizing different chemical admixtures or superplasticizers, ordinary Portland cement based rapid repair material can be set and develop early strength within a short period of time, e.g. less than 24 h. While it is suitable for rapid repair works, it should be noted that the early strength development of this type of material is temperature dependent and the physical strength development rate may decrease significantly as the surrounding temperature falls.
Alkali-activated cement (AAC), or geopolymer, is an environmentally friendly and quick-setting inorganic binding material with high early strength, excellent fire and acid resistance. It gains strength and other mechanical properties via chemical reaction between a source of alkali (soluble base activator) and calcium and/or aluminate rich materials. The alkali used as the activator is usually in solution form, and may include sodium/potassium silicate solution, sodium/potassium hydroxide solution, their combinations, or other sources of alkali. The formation of cementitious material involves a heterogeneous chemical reaction under alkaline conditions on silicon-aluminum minerals that gives a low-temperature sodium aluminosilicate glass, which is a special class of hydrated aluminosilicates chemically similar to naturally occurring zeolites. Having lower embodied energy and carbon footprints than ordinary Portland cement, alkali-activated cement is a prominent alternative cementing material for sustainable development.
However, one disadvantage of using liquid alkali as activators in the preparation of alkali-activated cement is the potential occupational and safety hazard problem since severe injuries such as chemical burns may occur during the concrete manufacturing process. Other disadvantages include the inconvenience of material storage, increased cost of transportation, possible leakage of alkali solution, etc. Therefore, there is a need to develop a dry-mix AAC with all constituents in powder form, one-part geopolymer cement (OPgC), which can be used on site by adding water only to gain strength and possesses superior durability over ordinary Portland cement.
The AAC formulated in dry-mix powder form can be further strengthened by nano modification utilizing nano-sized building blocks. Viewed from the bottom-up, alkali-activated cement based rapid repair material at the nanoscale is a composite of molecular assemblages, surfaces (binding matrix, aggregates and fibers, etc.), and chemical bonds that interact through local chemical reactions, intermolecular forces, and interphase diffusion. Processes occurring at the nanoscale, including origination and development of amorphous and crystalline phases and of interphase boundaries, ultimately affect the engineering properties and performance of the bulk material. Nano-modification or nano-engineering of the rapid repair material encompasses the techniques of manipulation of the material structure at the nanometer scale. It is believed that alkali-activated cement, or geopolymer, based repair material can be nano-engineered by the incorporation of nano-sized building blocks or objects, such as nanoparticles, to enhance its overall performance including workability, strength development and chemical stability and durability.